Percentage of ingested sodium excreted in 24-hour urine collections: a systematic review and meta-analysis

24 hr. urine sodium excretion is generally regarded as the ‘gold’ standard for assessing dietary sodium in population and epidemiological studies. This review examines the percentage of dietary sodium that is excreted in a 24 hr. urine collection. We systematically searched for all studies where a known and constant amount of dietary sodium was ingested for a minimum of 3 days and where sodium excretion in 24 hr. urine collections was measured. Studies with ‘healthy’ adult participants, or participants with health risks such as hypertension, were considered. 5264 unique studies were identified in the search; 392 underwent a full-text review and 35 studies were included. The pooled estimate for the percentage dietary sodium excreted in urine was 92.8% (95% confidence interval 90.7, 95.0) with little to no differences in subgroup analyses. There was high heterogeneity between studies, indicating that caution is required in interpreting the average percentage excretion, however, lack of study methodological rigor is likely to have contributed to the high heterogeneity. Although the generally high variability in sodium intake indicates that results from a single 24 hr. urine collection should not be used to assess an individual’s usual long-term sodium intake, this meta-analysis suggests that it is an appropriate method for assessing average dietary sodium in a healthy population or people with chronic stable health risks.